The strain gage load cell is one of the more common means of measuring force. It is a precisely machined structure which responds with a deformation to a given applied force. A strain gage bonded to a deformable element exhibits a change in resistance according to the degree of deformation, which results in an electrical signal indicative of the magnitude of the applied force. However, since the strain gage is bonded directly to the load cell, it does not permit rotation of the load cell with respect to the sensing electronics. Unless slip rings or some form of isolated electronics are utilized, it is impossible with a strain gage load cell to measure axial loads on a rotating shaft. Moreover, strain gage sensors are very expensive and are thus commercially impractical for competitive use in many load cell applications.
It is well known that the axial deflection of a helical spring produces a torsional strain in the spring material. For this reason, when a solenoidal coil spring constructed of a magnetoelastically active steel and circumferentially magnetized around the axis of the wire forming the coil is placed under axial loading, the coil wire twists, causing a reorientation of the magnetization in the wire, with the magnetization becoming increasingly helical as the axial loading increases. As a result, the helical magnetization has both a circumferential component and an axial component, i.e., parallel to the wire in the coil. The axial component of magnetization causes a magnetic field to arise which extends in a direction parallel to the axis of the coil spring. This magnetic field can be sensed and an electrical signal developed which ideally should be proportional to the magnitude of the axial loading. However, due to the forming process of straight wire into a coil spring, internal residual stresses are created within the coil which develop their own magnetic fields under stress and which contribute to the net field sensed by the magnetic field sensors. As a result, the electrical signal which is developed is not solely a function of the applied axial load but also reflects the contribution of the internal residual stresses developed during processing. Accordingly, such solenoidal coil springs do not provide inherently accurate and reproducible results and do not represent a good choice for measuring axial loading, such as in load cells.